This invention relates to electrochromic displays and particularly to an electrochromic display having an improved electrolyte.
The prior art discloses various electrochromic materials which may exist in more than one oxide state and which each exhibit different optical properties. Typical examples are set forth in U.S. Pat. No. 2,319,765 to Talmey which issued May 18, 1943 and U.S. Pat. No. 3,521,941 to Deb et al. which issued July 28, 1970.
The use of electrochromic materials in various electrooptical displays is revealed in Jones U.S. Pat. No. 3,283,656 issued Nov. 8, 1966 and Beegle U.S. Pat. No. 3,704,057 issued Nov. 28, 1972.
The Jones patent discloses a coloring material as the electrolyte while the Beegle patent discloses a gelled electrolyte. Also of interest in this area of electrooptical displays are U.S. Pat. No. 3,451,741 to Manos issued June 24, 1969 and U.S. Pat. No. 3,807,832 to Castellion, issued May 18, 1971.
Further patents of interest include U.S. Pat. Nos. 3,827,784 and 3,840,287. U.S. Pat. No. 3,827,784 to R. D. Giglia et al. which issued Aug. 6, 1974 discloses an electrochromic device including an improved bonded graphite counter electrode which is deposited from a sample solution or suspension under ambient conditions.
U.S. Pat. No. 3,840,287 to Witzke et al. which issued Oct. 8, 1974 describes an electrochromic cell having a color promoting agent located between first and second layers of electrochromic material and provides an unbalanced arrangement wherein one of the electrochromic layers is initially in its colored state so that current can readily flow through the colored layer.
The pending application Ser. No. 540,481 of M. Leibowitz filed Jan. 13, 1975 which is assigned to the assignee of record of this application relates to an electrochromic display wherein a liquid electrolyte is used for ion transfer between spaced electrochromic layers. The present invention relates to a display of this general type having an improved electrolyte and the method of making said display. In the case where a particular oxide state of the electrochromic material is soluble in the electrolyte, there will be degradation of the electrochromic layers with time and temperature. The degree of solubility may differ with each oxidation state through which the electrochromic material passes. It would be desirable to provide an improved electrolyte which reduces the degradation of the more soluble oxidation states of the electrochromic layers and hence improves the life and visiblity of the overall display.
U.S. Pat. No. 3,819,252 issued June 25, 1974 to Giglia discusses electrochromic devices wherein a conductive electrolyte is saturated with the same compounds as those used in the imaging layer, alleged to result in faster switching time. However, the additive compounds suggested do not correspond with those associated with the greatest degradation of the image layers.